Electromagnetic interference (EMI) is a well known problem with electronic instruments having circuitry operating at high frequencies. Uncontrolled EMI from one instrument may interfere with the operation of other nearby sensitive electronic instruments. Accordingly, EMI has been governmentally regulated to limit the amount of tolerated EMI emissions.
To control, limit, or avoid EMI emissions, instruments are often shielded by use of metal housings, or by metal shield plates within a housing to surround critical circuitry. Even instruments operating a low frequencies that do not generate EMI may be sensitive to emissions by other instruments. In particular, hand held instruments are susceptible because a user acts as an antenna while touching an instrument. A particular concern is the 60 Hz background signal present in most indoor environments in which an instrument is likely to be used.
For effective shielding, the shield must be electrically connected to the circuitry; typically the shield is connected to the ground plane. However, the connection of shield plates to a node on a printed circuit board (PCB) presents difficulties. Normally, components are soldered to a PCB in a single efficient process such as wave soldering or reflow. To connect shields to a PCB node normally requires a subsequent manual operation, increasing manufacturing costs and inviting assembler errors. In one approach, simple coil springs are hand soldered to a PCB to protrude perpendicularly from the board. When the shield is installed, it presses against the springs, providing electrical contact. While effective, the manual process generates inefficiencies. Automation of the spring soldering process is impractical because the required length of the springs makes them unstable during soldering; widening the bases of the springs to provide stability is undesirable because it consumes valuable space on the PCB. Other shield contact options include conventional EMI leaf spring clips, conductive foam pads, and plated cloth adhered with conductive adhesive. All these options present manufacturing and/or reliability difficulties.
The embodiment disclosed herein overcomes these limitations by providing an electronic instrument having a housing containing a circuit assembly with a hole. A first shield in the housing is positioned adjacent the hole on one side of the circuit assembly, and a second shield is positioned on the opposite side of the circuit assembly. A spring has a first portion compressed between the circuit assembly and the first shield, and a second portion extending through the hole and biased against the second shield. The spring may be a coil spring with the first portion having a larger diameter to provide a shoulder to rest against the circuit, and the second portion having a smaller diameter to pass through the hole.